The present invention relates to a data transmission system for transmitting data over telephone lines or similar analog lines and, more particularly, to a jitter cancelling apparatus for carrier phase control associated with a receiver of such a data transmission system.
In a data transmission system, signals usually undergo various kinds of deterioration as typified by amplitude distortions, delay distortions, carrier frequency offsets and carrier phase jitters while being transmitted over lines. Among them, amplitude distortions and delay distortions are almost time-invariant or, if time-variant, the variation is slow enough to allow such distortions to be compensated for by so-called automatic equalizers. Carrier phase jitters, on the other hand, result in time-variant distortions and this kind of distortion has hitherto been absorbed by a phase locked loop (PLL).
The PLL scheme, however, cannot sufficiently suppress phase jitters (especially, high-frequency jitters) unless the frequency band of the PLL is broadened to the order of 200 hertz to 300 hertz. Such a broad loop band would deteriorate the noise characteristic of the PLL and thereby lower the resistivity to noise of the entire data transmission system.
Another implementation for the suppression of phase jitters heretofore proposed is a jitter canceller system as disclosed in U.S. Pat. No. 4,639,939. The jitter canceller system uses a predictive filter, which is tuned to phase jitters, so as to cancel phase jitters being superimposed on a carrier wave. This type of system can be implemented with a narrow-band PLL which is adapted to recover a carrier wave, thereby eliminating the deterioration of the resistivity to noise. However, a problem with the jitter canceller system is that the predictive filter has to be scaled up in inverse proportion to the lower limit of phase jitter frequency which is to be suppressed. Should the system be designed to cope with even the jitter whose frequency is as low as 20 hertz or so, the hardware scale would become excessively large and therefore impractical.